Most cells in your body have a “resting membrane potential” in which they’re ready to send an electrical signal but they need a trigger to tell them when to fire. The pacemaker cells in the heart don’t have a resting membrane potential, they send that signal as soon as soon as the cell reaches threshold potential. The charge needed to send the signals is dependent on the movement of electrolytes so that is what determines how quickly the heart beats.
The best analogy I can think of is one of those bamboo shoots that fills with water and then tips over when it’s full. Think less water when you’re relaxed and more water when you’re excited. The rate may change but the rhythm will remain constant.
Sorry if it got too medical, I don’t think you could explain this to a 5-year old lol
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